An Optical Network (ON) is comprised of a plurality of optical nodes linked together to form a network. Lightpaths are optical connections carried over a wavelength, end to end, from a source node (also known as a head-end node) to a destination node (also known as a far-end node) in the optical network (ON). The ON includes a data layer, a digital layer, and an optical layer. The optical layer contains multiple sub-layers. ON structure, architecture, and modeling are further described in the International Telecommunication Union recommendations, including ITU-T G.709, ITU-T G.872, and ITU-T G.805, which are well known in the art. In general, the ON is a combination of the benefits of the digital connectivity layer, may it be e.g. SONET/SDH or Ethernet based technology, and dense wavelength-division multiplexing (DWDM) technology (optics, Layer 0, the Physical Layer).
Wave-division multiplexing (WDM) is a type of multiplexing in which two or more optical carrier signals are multiplexed onto a single optical fiber by using different wavelengths (that is, colors) of laser light.
Different WDM systems may be referred to as normal wave-division multiplexing (WDM), coarse wave-division multiplexing (CWDM) and dense wave-division multiplexing (DWDM), depending on their wavelength patterns. ITU-T has standardized spectral grids for course WDM (CWDM) in Recommendation G.694.1 and dense WDM (DWDM) in Recommendation G.694.2.
To successfully communicate over the Optical Network (ON), the physical interfaces of the end nodes need to follow certain criteria. ITU-T has defined physical layer interfaces for multivendor interoperable systems in G.959.1. The ITU-T standard G.695 defines physical layer interfaces for multivendor interoperable CWDM systems. The ITU-T series of standards G.698.x defines physical layer interfaces for multivendor interoperable DWDM systems. More specifically, G.698.3 defines a system where the transmitters use a seed signal to lock to the desired transmission wavelength. This standard currently only defines applications for 1.25 Gb/s bit rates.
Tunable DWDM laser transmitter technology is widely used in the metro/core part of optical communication networks and is regarded as a good long term transmitter technology in the access portion of networks. Tunable lasers may reduce the inventory of transceivers to a single type of transceiver (rather than a plurality of transceivers that are pre-set to a single wavelength) and provide excellent optical transmission properties. Today, tunable dense wave-division multiplexing small form-factor pluggable (DWDM SFP+) optical modules represent transceiver technology available in the market for 10 Gb/s and higher data rates applications.
However, one problem with the use of tunable transmitters in the access part of the network is that they need to be configured to transmit at the correct wavelength (channel) corresponding to the wavelength port of the wave-division multiplexing (WDM) filter to which they are connected. Typically, the equipment that sits in the link end of the network that is closest to the end customer (i.e., the far-end side) is unaware of what wavelength on which to transmit. This could also apply to the equipment that sits closest to the core of the network (i.e., the head end side).
The term “colorless” may be used to describe the ability to adapt the wavelength that is transmitted to the filter port on the WDM filter to which an optical networking unit (ONU) is connected. Such colorless behavior could be implemented by using a reflective source. One example is the ASE injected Fabry-Perot semiconductor laser diode. This technology was first commercialized by Novera Optics in the beginning of the 2000's. Transmode Systems AB (now Infinera) also commercialized a system based on this technology (iWDM-PON released in 2012). However, this technology may be technically challenging and costly to scale to 10 Gb/s and higher bit rates due to the intrinsic relative intensity noise (RIN) degradation of the optical signal as the bandwidth of the semiconductor laser device is increased. By using tunable lasers, 10 Gb/s and higher bit rates is technically feasible, but these sources lack the colorless ability that comes with the ASE injected Fabry-Perot lasers.
Monolithically integrated tunable lasers are widely used in the core and metro part of the networks, but have been used to a very limited extent in the access networks. A technical problem with optical modules based on tunable laser technology is the lack of ability to auto-tune the laser's wavelength. One proposed solution to this problem uses pilot tones to assist tuning. For example two technical papers describe a method for auto-tuning using pilot tones (they highlight different aspects of the same method): M. Roppelt, F. Pohl, K. Grobe, M. Eiselt, J.-P. Elbers, “Tuning Methods for Uncooled Low-Cost Tunable Lasers in WDM-PON”, in Proc. OFC, Paper NTuB.1, 2011; and M. Roppelt, M. Eiselt, K. Grobe, J.-P. Elbers, “Tuning of an SG-Y branch laser for WDM-PON”, in Proc. OFC, Paper OW1B.4, 2012. However, a problem with such an approach is that it adds hardware cost and complexity and negatively impacts the performance of the optical signal.
What is needed is an automatic tuning mechanism of the tunable lasers in the system, since this would greatly reduce the operational complexity and cost for the operator. The systems and methods disclosed herein solve the problem at a low cost and complexity and avoid penalty to the optical signal.